The present invention relates generally to copolymers useful in applications such as drug delivery, protein separation, and gene delivery vectors. The copolymers are water soluble, pH-sensitive and capable of thermoreversible gelation.
One beneficial characteristic that polymeric materials have added to the field of drug delivery is their ability to respond to their environment. By modifying the chemical composition of either the backbone or pendant groups, polymers can respond to a wide range of stimuli. One stimulus more often exploited for drug delivery purposes is pH dependence. This dependence can be expressed by polymers having either anionic or cationic characteristics. Polymers with cationic functionality will tend to swell in low pH aqueous solutions whereas polymers with anionic functionality tend to swell in high pH solutions.
Cross linked cationic polymer membranes of diethylaminoethyl methacrylate (DEAEM) and dimethylaminoethyl methacrylate (DMAEM) have been previously synthesized for drug delivery applications. (See G. Albin et al., J. Controlled Rel., 2:153 (1985); J. Kost et al., J. Biomed. Mater. Res., 19:1117 (1985); K. Ishihara et al., Polymer J., 16(8):625 (1984); D. Hariharan and N. A. Peppas, Polymer., 37(1):149 (1996); and L. M. Schwarte and N. A. Peppas, Polymer., 39(24)L6057 (1998).) In these cases, the polymers have been rendered glucose sensitive by the attachment of glucose oxidase, thus providing a possible vehicle for insulin release. One main disadvantage of these materials, however, is that they are not water-soluble and, if implanted, remain in the body long after the useful life of the delivery device.
Other research efforts have focused on graft and block copolymers comprised of domains with anionic functionality and separate water-soluble portions, such as poly(ethylene glycol) (PEG). (See A. S. Hoffman et al., Polym. Prepr., 38(1):524 (1997); L. Bromberg, Ind. Eng. Chem. Res., 37:4267 (1998); and L. Bromberg, J. Phys. Chem., 102:1956 (1998).) These polymers have been primarily used for the release of drugs in the intestines, where a rise in pH would indicate that the device had passed through the stomach and is no longer in the harsh acidic conditions. Once in the intestines, where the pH is higher, the delivery polymer then becomes water-soluble and the polymer-bound drug may be released.
Nagasaki et al. also reported the production of various methacrylic block polymers with possible use in drug delivery applications. (Nagasaki et al., Macromol. Rapid. Commun., 18:827 (1997).) These block polymers are prepared by polymerizing a methacrylic ester monomer, having an electron-donating substituent group bonded to a specific site of its ester residue, using a potassium alcoholate. Under this polymerization system, if a cyclic ether (e.g., ethylene oxide) or a cyclic ester (e.g., a lactide or lactone) is reacted with the methacrylic ester monomer, a living polymer chain could be produced with the methacrylic ester extending through the medium of the living polymer chain. If the cyclic ether or cyclic ester is allowed to coexist in the reaction system, the monomer may also readily undergo co-polymerization to yield a block copolymer of a methacrylic ester possessing both a functional group at the ester sides and a lactide or lactone.
Other polymers, such as polyethyleneimine, have been used as vehicles to induce flocculation of proteins and other biomacromolecules. (See Mortimer, D. A., Polymer Inter., 25:29 (1991); and Chen et al., Chem. Eng. Sci., 47:1039 (1992).) In these polymers, the electrostatic interaction between the biomolecule and the selected polyelectrolyte provides the means to selectively precipitate charged molecules out of an aqueous solution, such as a fermentation broth.
The newest use for cationic polymers is the delivery of genetic material to mammalian cells for gene therapy applications. Recently van de Wetering et al. outlined the use of tertiary amine methacrylate homopolymers for gene delivery and the effect that the type of methacrylate has on the transfection efficiency. (See van de Wettering et al., J. Controlled Release, 64:193 (2000).) It was found in this study that a homopolymer of 2-(diethylamino)ethyl methacrylate (DEAEM) might be a useful delivery material for plasmid DNA. However, it was also found that DEAEM could not form polymer/DNA complexes like many other cationic methacrylates, presumably because of the low water solubility of the polymer.
Rungsardthong et al., also recently reported the use of copolymers for gene delivery applications. (Rungsardthong et al., J. Controlled Release, 73:359–380 (2001).) In this study copolymers of DMAEMA with poly(ethylene)glycol (PEG) were investigated for their ability to serve as vectors in gene therapy. In vitro transfection experiments in this study showed that the DMAEMA homopolymer gave the highest level of transfection as compared to the control poly-L-lysine (PLL) system. The PEG:DMAEMA copolymer gave reduced levels of transfection, believed to be due to the steric stabilization effect of the PEG corona.
One area which has not yet been fully exploited is the use of pH-dependent functionalities in non-crosslinked injectable systems. Such systems would have advantages over crosslinked systems as they can be simply injected into the body to form a solid non-crosslinked gel that will eventually dissolve and be excreted.
Crosslinked hydrogels incorporating characteristics of pH and/or temperature sensitivity for stimuli-sensitive release of pharmaceutical drugs have only recently been developed. (See Lowman et al., J. Pharm. Sci., 88:933 (1999); and Brazel, C. S., and Peppas, N. A., Macromolecules, 28:8016 (1995).) For example, the triblock copolymer Pluronic® (poly(ethylene oxide)-b-poly(propylene oxide)-b-poly(ethylene oxide)) has distinct amphiphilic properties and the ability to form non-crosslinked gels. Under the appropriate concentration and thermal conditions, aqueous solutions having this polymer will form micellar systems consisting of dehydrated poly(propylene oxide) cores surrounded by solvated poly(ethylene oxide) coronas. The segregated lipophilic nanophase of this compound can also increase the total aqueous solution solubility of small organic molecules like naphthalene and ibuprofen, molecules that are relatively insoluble in non-micellar aqueous solutions. At sufficient polymer concentrations, the Pluronic polymer will also undergo a sol-gel transition at temperatures slightly higher than its critical micellization temperature (CMT). When water penetrates the gel, lowering the total concentration of the polymer at the gel interface below a concentration sufficient to maintain the gel state at that temperature, the gel will dissolve to cause a controlled release of the associated pharmaceutical drug.
However, the Pluronic® polymers are not sensitive to pH and typical in vitro dissolution times have been on the order of 5–6 hours. (See Chi, S. C., and Jun, H. W. J. Pharm. Sci., 80:280 (1991); and Anderson et al., Journal of Controlled Release, 70:157 (2001).) Although in vivo release times are slightly longer, on the order of 10–20 hours, the Pluronic polymers on their own may not be extremely useful for controlled drug or bioactive molecule delivery. In particular, to compete with the commercially available orally administered controlled released tablets, injectable devices must release their dosage over a time period much longer than that available with the present Pluronic® system.